Many published studies show the benefits of forced pulsation of the liquid flow rate in a trickle bed reactor, especially when mass transfer phenomena are rate controlling. Two types of periodic liquid pulsation, the slow and fast mode, are examined in the literature, depending on the period of the pulsation. The aim of this work is to show that, when slow mode pulsation occurs, the quasi steady-state model can be applied. The model assumes that the mean conversion rate in pulsed mode is the weighted mean of the two steady conversion rates at the minimum and maximum liquid flow rates. In fact, the period of the liquid pulsation is long enough to allow the reactor to reach the quasi steady-state conditions at every level of the liquid flow rate. The validity of the model is checked with experimental data concerning alpha-methylstyrene hydrogenation to cumene.